1. Field of the Invention
Aspects of the present invention relate to a rechargeable lithium battery having improved cycle-life characteristics.
2. Description of the Related Art
Rechargeable lithium batteries have recently drawn attention as power sources for small portable electronic devices. Rechargeable lithium batteries use an organic electrolyte solution, have a high energy density, and have a discharge voltage twice as high as conventional batteries that use an alkali aqueous solution.
Rechargeable lithium batteries can have a positive active material, for example a lithium-transition element composite oxide capable of intercalating lithium. For example, LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2 (0<x<1), LiMnO2, and the like, have been researched.
Rechargeable lithium batteries can have a negative active material namely, various carbon-based materials. For example, artificial and natural graphite, and hard carbon, all of which can intercalate and deintercalate lithium ions. The use of graphite increases a discharge voltage and the energy density of a battery, because graphite has a relatively low discharge potential of −0.2V, as compared to lithium. A battery using graphite as a negative active material has a high average discharge potential of 3.6V and an excellent energy density. Furthermore, graphite is the most comprehensively used of the aforementioned carbon-based materials, since graphite guarantees better cycle-life characteristics for a battery, due to its outstanding reversibility. However, a graphite active material has a low density, and consequently, a low capacity (theoretical capacity: 2.2 g/cc), in terms of energy density per unit volume, when used as a negative active material. Furthermore, the use of a graphite active material entails possible dangers when a battery is misused, overcharged, and the like, such as, explosion or combustion, since graphite is likely to react with an organic electrolyte, at a high discharge voltage.
In order to solve these problems, a great deal of research on oxide negative electrodes has recently been performed. For example, amorphous tin oxide, developed by Japan Fuji Film Co., Ltd., has a high capacity per weight (800 mAh/g). However, this oxide has resulted in some critical deficiencies, such as having a high initial irreversible capacity of up to 50%. Furthermore, amorphous tin oxide has a discharge potential of more than 0.5V and shows a smooth voltage profile, which is unique to the amorphous phase. Consequently, it is difficult to establish a tin oxide that is applicable to a battery. Furthermore tin oxide tends to be reduced to tin, during a charge/discharge reaction, which is a disadvantage for use in a battery.
Referring to another oxide negative electrode, a negative active material of LiaMgbVOc (0.05≦a≦3, 0.12≦b≦2, 2≦2c−a−2b≦5) is disclosed in Japanese Patent Publication No. 2002-216753. A lithium secondary battery including Li1.1V0.9O2 an oxide negative electrode was also presented in the 2002 Japanese Battery Conference (Preview No. 3B05). Such oxide negative electrodes do not show sufficient battery performance, and therefore, there has been a great deal of further research into oxide negative active materials.
Referring to yet another oxide negative electrode, a negative active material of a vanadium oxide not including Li was disclosed in Solid State Ionics, 139, 57-65, 2001 and Journal of Power Source, 81-82, 651-655, 1999. However, this active material has a different crystal structure from the active materials of the present invention. In addition, since the active material has an average discharge potential of more than 1.0 V, it may have problems when used as a negative electrode.